This proposal will investigate central motor control mechanisms that underlie maintenance of posture. Central control of posture is expressed through anticipatory postural adjustments (APAs) that precede planned postural perturbations and minimize them with anticipatory corrections, and compensatory postural adjustments that deal with actual perturbations of balance that occur because of the suboptimal efficiency of anticipatory corrections. Recently, the Principal Investigator has developed an experimental paradigm for the investigation of anticipatory postural adjustments during self-initiated releasing of a load while standing. The paradigm allows the separation of three major factors that influence APAs: motor action, the effects of external perturbations, and postural stability. Six hypotheses clarifying the organization of anticipatory postural adjustments will be tested in eight experiments. Specific aim 1 will examine the central organization of APAs; Specific aim 2 will independently investigate the effect of each of two factors, motor action and the magnitude of perturbation upon anticipatory postural adjustments. Specific Aim 3 will investigate the effect of postural instability on anticipatory postural adjustments. Specific aim 4 will examine the effect of the relation between motor action and perturbation on anticipatory postural adjustments. It is felt that by understanding the nature of the anticipatory postural adjustments in normal adults one will be better able to understand the loss of control of posture in patients with pathology. Clinical application of the results of the study will define more precisely the focus of conventional rehabilitation strategies aimed at improving postural control in people with postural deficits.